Recently, in tires for heavy duty and high speed use such as aircraft tires, a radial ply carcass has become used for structural durability, running performance and low fuel consumption.
For such aircraft tires, however, since they are used under conditions of high internal pressure, heavy load and high speed, a greater durability is required compared to tires used in other fields. Especially the bead is likely to be peeled apart or damaged in the carcass end part and its vicinity due to the strain caused by bending deformation by large loads at the time of taking off or landing.
Therefore, it has hitherto attempted to suppress bending deformation by enhancing the bead rigidity by increasing the bead volume by using reinforcement layers made of organic or inorganic fiber cords.
In such rigidity reinforcing measures, however, the internal heat generation due to bending deformation could not be decreased, and satisfactory preventive effects of bead portion damages were not obtained.
The present inventors accordingly studied the bending deformation of the bead portion by carrying out take-off taxi simulation tests and other similar tests conforming to the Federal Aviation Administration Technical Standard Order TSO-C62c. As a result, comparing the tire bead portion A0 without load and the tire bead portion A1 with specified load as shown in FIG. 2, mounted on its specified rim R and inflated to its specified internal pressure, it was found that the internal heat generation of the bead portion is lower when the bead deformation h0-h1 is smaller, where h0-h1 is the difference between the initial height h0 and the loaded height h1, i.e. the length between the outer surface of the beads A0 and A1 on the vertical line set up from the outer edge of the Phn flange Ra, and the upper edge of the rim flange Ra as shown in FIG. 3.
The present inventors carried out further research, and discovered that by employing high modulus rubber as a bead apex, the loaded height h1 is increased without practically changing the initial height h0, and thereby the bead deformation h0-h1 is decreased, and that by decreasing the modulus from the inside to the outside in the tire axial direction, the shearing stress occurring in the bead in the loaded state can be lessened to enhance the bead durability.